From the state of the art, a magnetic absolute position sensor is known from the documents DE 10 2007 039 051 A1 and DE 10 2011 002 179 A1.
The document DE 10 2007 039 051 A1 describes a rotatory absolute position sensor, which is capable to determine on the one hand an angular posture of a permanent magnet and, on the other hand, to count a number of revolutions of the permanent magnet as well as to store a value, which corresponds to said number, in a non-volatile memory. The absolute posture of the permanent magnet may be detected from the value, which corresponds to said number, and the current angular posture.
If an outside energy supply is at least temporarily not available, the described sensor is capable to continuously keep counting the number of revolutions and to store the number in the non-volatile memory. The sensor receives the energy, which is necessary for this, from a Wiegand module, which provides voltage impulses in defined temporary intervals as a function of the frequency of the revolutions of the permanent magnet, wherein the voltage impulses are used, beside the energy supply, for counting the revolutions.
As long as the outside energy supply is not available, the angular posture of the permanent magnet is not determined.
If the outside energy supply is switched on again and/or if it is available again, the angular posture of the permanent magnet may be determined immediately.
For obtaining the absolute position of the permanent magnet again, it is necessary to synchronize the value, which is stored in the non-volatile memory, with the angular posture.
In this connection, the following difficulty exists.
If, after the generation of the voltage impulse by the Wiegand module, a change of the movement direction of the permanent magnet is effected, the risk exists that the next voltage impulse, which would have to occur regularly, is rudimentary, and therefore is not recognized. If, afterwards, the sensor and/or the permanent magnet comes to a standstill in a particular (unfavourable) angular range, and if the outside energy is switched off, a synchronization and a resumption of the operation of the sensor cannot be performed reliably upon re-establishment of the outside energy supply, because there is no unambiguity (or uniqueness) about via which way (or path) the permanent magnet has come to its last posture.
The developed ambiguity could be lifted by further moving the permanent magnet further, and by sensing the next voltage impulse produced by the Wiegand module. However, it is not always possible to move the permanent magnet further. In this case, it is necessary to find out in another way via which way (or path) the permanent magnet has come to its current posture, in order to establish an error-free position sensor.
In this connection, the patent document DE 10 2011 002179 A1 proposes to evaluate the magnetization direction of the Wiegand wire in order to obtain information about the path of movement.
This is performed, on the one hand, by a magnetic sensor, which is arranged in the vicinity of the Wiegand wire, and which detects the magnetization direction of the Wiegand wire.
On the other hand, the possibility exists to supply the coil, which is wound around the Wiegand wire, with current, in order to achieve a reversal of magnetism of the Wiegand wire, and to evaluate the strength of current, which is necessary for this.
However, both variants have disadvantages.
The remanence (or residual magnetism) of the Wiegand wire is very small, such that the magnetic sensor has to be very precise in order to be able to detect the magnetization direction of the Wiegand wire.
In contrast, the supply of the coil, which is wound around the Wiegand wire, with current causes additional component parts and thus costs.